The present invention relates to the long-term storage of spent fuel that has been removed from a nuclear reactor, and more particularly, to a spent fuel storage cask having improved fins for dissipating heat generated by the spent fuel.
FIG. 1 illustrates a typical fuel assembly 20 for supplying nuclear fuel to a reactor. Assembly 20 includes a bottom nozzle 22 and a top nozzle 24, between which are disposed elongated fuel rods 26. Each fuel rod 26 includes a cylindrical housing made of a zirconium alloy such as commercially available "Zircalloy-4", and is filled with pellets of fissionable fuel enriched with U-235. Within the assembly of fuel rods 26, tubular guides (not shown) are disposed between nozzles 22 and 24 to accommodate movably mounted control rods (not illustrated) and measuring instruments (not illustrated). The ends of these tubular guides are attached to nozzles 22 and 24 to form a skeletal support for fuel rods 26, which are not permanently attached to nozzles 22 and 24. Grid members 28 have apertures through which fuel rods 26 and the tubular guides extend to bundle these elements together. Commercially available fuel assemblies for pressurized water reactors include between 179 and 264 fuel rods, depending upon the particular design. A typical fuel assembly is about 4.1 meters long, about 19.7 cm wide, and has a mass of about 585 kg., but it will be understood that the precise dimensions vary from one fuel assembly design to another.
After a service life of about three years in a pressurized water reactor, the U-235 enrichment of a fuel assembly 20 is depleted. Furthermore, a variety of fission products, having various half-lives, are present in rods 26. These fission products generate intense radioactivity and heat when assemblies 20 are removed from the reactor, and accordingly the assemblies 20 are moved to a pool containing boron salts dissolved in water (hereinafter "borated water") for short-term storage. Such a pool is designated by reference number 30 in FIG. 2.
Pool 30 is typically 12.2 meters deep. A number of spent fuel racks 32 positioned at the bottom of pool 30 are provided with storage slots 34 to vertically accomodate fuel assemblies 20. A cask pad 36 is located at the bottom of pool 30.
During the period when fuel assemblies 20 are stored in pool 30, the composition of the spent fuel in rods 26 changes. Isotopes with short half-lives decay, and consequently the proportion of fission products having relatively long half-lives increases. Accordingly, the level of radioactivity and heat generated by a fuel assembly 20 decreases relatively rapidly for a period and eventually reaches a state wherein the heat and radioactivity decrease very slowly. Even at this reduced level, however, rods 26 must be reliably isolated from the environment for the indefinite future.
Dry storage casks provide one form of long-term storage for the spent fuel. After the heat generated by each fuel assembly 20 falls to a predetermined level--such as 0.5 to 1.0 kilowatt per assembly, after perhaps 10 years of storage in pool 30--an opened cask is lowered to pad 36. By remote control the spent fuel (either in the form of fuel assemblies 20 or in the form of consolidation canisters which contain fuel rods that have been removed from fuel assemblies in order to increase storage density) is transferred to the cask, which is then sealed and drained of borated water. The cask can then be removed from pool 30 and transported to an above-ground storage area for long-term storage.
FIG. 3 illustrates a sectional view of a typical storage cask 38. Cask 38 includes a cask base element 40 having a floor 42 and a hollow interior provided by cylindrical walls 44. Although not illustrated, the hollow interior houses a fuel support matrix which provides an array of vertically oriented storage slots for receiving spent fuel and which transfers heat generated by the spent fuel to walls 44 for subsequent dissipation into the environment. Cask base element 40 includes a carbon steel portion 46 which is approximately 25 cm thick and which serves to protect the environment from gamma rays. Portion 46 is surrounded by a layer about 7.0 cm thick of neutron absorbing material 48, which may be a resin. Surrounding material 48 is an outer layer 50 of stainless steel to protect cask 38 from the environment. Cask 38 also includes a cask lid element (not illustrated) which is bolted to base element 40 in order to seal the cask after it is loaded with spent fuel. Like base element 40, the cask lid element has a thick carbon steel portion, a neutron absorbing layer, and an outer layer of stainless steel.
With continuing reference to FIG. 3, cask base element 40 includes carbon steel cooling fins 52, which are welded to portion 46 and which extend through material 48 and layer 50. Fins 52 are elongated and have axes that are parallel to the axis of base element 40. Fins 52 are present to conduct heat through material 48, which is not a good heat conductor, and convey it to the environment by means of convection and infrared radiation. Efficient heat removal is essential since the temperature of the fuel rods 26 within cask 38 must be kept below a maximum temperature, such as 375.degree. C., to prevent deterioration of the zirconium alloy housing.
Cask 38 is typically about 4.8 meters high and has an outside diameter of about 2.5 meters, excluding the cooling fins. It has a mass of over a hundred thousand kilograms when loaded with spent fuel. Due to the mass and size of cask 38, it will be apparent that fins 52 are subject to damage as a result of rough treatment or accidents during handling and transportation of the cask.
It is desirable to treat fins 52 in order to protect the carbon steel from chemical attack by the environment. In the past this protection has been supplied by weld-depositing stainless steel ribbons about 2.5 cm wide on the side surfaces 54 of the carbon steel. This is relatively expensive, however, and moreover creates heat distortion and otherwise mars the appearance of the surface of the fins. Furthermore it is difficult to deposit stainless steel to protect the edges 56 of fins 52.